In a next generation radio communication system for which a high-speed transmission of 100 M through 1 G bit/sec. is requested, it is assumed that a high frequency band is assigned. However, a high frequency band signal generally has higher rectilinearity than a low frequency band signal, and it is known that there occur a number of dead zones where radio waves cannot travel. Therefore, if it is assumed that the transmission power of a base station is equal to that of the currently commercialized radio communication system, the assignment of a high frequency band reduces the coverage of cells (service area). It is not preferable not only in rise of cost by an increasing number of base stations but also in occurrence of frequent handover.
Under the circumstances, there is a radio communication system proposed by implementing a relay station for relaying radio communications between a base station and a mobile station. Generally, since a relay station is less costly than a base station, the entire system can be realized at a low cost while maintaining sufficient coverage by implementing the relay station. The radio communication system provided with the relay station (hereinafter referred to as a relay system) is disclosed in, for example, the following non-patent document 1. The relay system is also surveyed in the LTE (Long Term Evolution)-advanced as disclosed in, for example, the following non-patent document 2.    Non-patent Document 1: 802. 16j-06/026r4, Air Interface for Fixed and Mobile Broadband Wireless Access Systems: Multihop Relay Specification, Jun. 6, 2007.    Non-patent Document 2: China Mobile, Vodafone, and Huawei, “Application Scenarios for LTE-AdvancedRelay”, R1-082975, Aug. 18-22, 2008.
In the relay system, it is assumed that data is transmitted through radio waves along a downlink from a base station to a mobile station through a relay station. In this case, when the mobile station is located relatively near the relay station, the radio transmission quality using the relay station is generally better than in the radio transmission of data from the base station directly to the mobile station. First, the propagation environment between the base station and the relay station enables a LOS (Line Of Sight) communication, and communications can be performed with high directivity using the beam forming technology from the base station to the relay station with a fixed arrangement. Second, the propagation environment from the relay station to the mobile station is better than that from the base station to the mobile station.
Accordingly, to improve the throughput of the entire relay system, it may be selected to adopt an appropriate modulation and coding scheme by an adaptive modulation and coding scheme (AMCS) depending on the better propagation environment from the relay station to the mobile station when data transmission is performed from the relay station to the mobile station.